Bone is subject to continuous remodeling throughout life. The age-related loss of (trabecular) bone, leading to senile osteopenia, is mainly due to impaired bone formation. Osteoblasts (OB) and osteoclasts (OC) have been identified as playing a crucial role in the process of bone turnover, but the contribution made by their precursors is not well documented. We analyzed the cells of the osteoblast and osteoclast cell lineage along the trabecular bone of tibiae and the stromal cells in the marrow of aging BN/Bi Rij rats using electron microscopy. It appeared possible to distinguish preosteoblasts (pre-OB), OB, preosteoclasts (pre-OC), OC, and inactive bone-lining cells. Periods of increase, the maximal peak, and the decrease in trabecular bone volume were defined by means of morphometric measurements of trabecular bone volume. We found a decrease of more than 10-fold in the number of OB with age, but the numbers of pre-OB, pre-OC, and OC expressed per unit bone length, although variable, were age independent. The relative bone resorption and formation surface, expressed as a percentage of the total bone surface, decreased 2- and 15-fold, respectively. In 2-year-old animals the total volume of stromal cells, part of which constitutes the stem cell compartment of the osteogenic lineage, was a quarter of that found in 1-month-old animals and a third of that found in 6-month-old animals. The loss of trabecular bone is concomitant with a sharp increase in the ratio of pre-OB/OB, the ratio of OC/OB, and in the ratio of resorption to formation surfaces. There was no relation between the ratio of pre-OC/OC with age. These data lead to the conclusion that the main factor causing bone loss with age is a diminished maturation of pre-OB into OB.